I found the following in an article recently.

Quote:

"The intensity E (known as the field strength) of a transmission
at a distance D from a source transmitting P watts of RF power via
a half-wave dipole in a free, unobstructed space, can be estimated
using the formula:

E=(7*sqrt (P))/D.

Thus, for a 2 watt transmitter, the theoretical field strength in free
space at one metre distance is approximately 10 V/m and at 100 metres
distance 0.1 V/m"

This formula is new to me and I can't find it in any
textbooks I have.  Since it ignores frequency shouldn't
it be power flux per unit area and not field strength?
Can anyone tell me where it comes from?

Walter G3JKV.